Chassis parts of automobiles, such as a steering knuckle and suspension upper arms, and hydraulic parts of construction machines, such as a piston rod end, require both high strength and high toughness. Conventionally, in order to meet such mechanical requirements, those parts are made of middle carbon steels such as JIS-S43C, S45C, S48C, etc. (roughly they correspond to SAE 1042, 1045, 1049 steels), and, after hot forging, they are heat treated; i.e., heated, quenched (hardened) and reheated for tempering. The heat treatment should be carefully performed or, without appropriate heat treatment, these steels cannot exhibit proper performances.
One problem about the conventional heat-treatment middle carbon steels is that complete heat treatment is impossible for parts having a large mass (or a large cross section); parts having a cross sectional area larger than 10,000 mm.sup.2 cannot be thoroughly quenched (hardened) to the core. Thus neither high strength nor high toughness can be obtained for such large parts.
Another and more serious problem about the heat-treatment steels is that the heat treatment consumes a large amount of energy. Recent social demand for less energy consumption has urged the development of so-called non-heat-treating steels which can provide mechanical properties required to such parts with only air cooling after they are shaped by hot forging. A typical non-heat-treating steel is a middle carbon steel with carbon content of 0.20-0.50% and vanadium content of 0.03-0.20%. When the steel is air cooled after hot forging, fine carbo-nitrides of vanadium precipitate in the ferrite matrix, which strengthen the ferrite matrix without later heat treatment.
The prior art non-heat-treating steels have strength comparable to heat-treated middle carbon steels. But the toughness is not comparable to heat-treated steels because the microstructure is coarse ferrite-pearlite when they are air cooled after hot forging. Another problem of the prior art non-heat-treating steels is that the requirements of hot forging conditions (e.g., heating temperature before forging, forging temperature, cooling speed after forging, etc.) are rather strict to obtain proper mechanical properties. Thus rather tedious preliminary tests are indispensable to determine an appropriate forging condition, and, when the forging starts, the forging conditions should be carefully controlled from time to time.
Further development in this field is a low-carbon bainitic non-heat-treating steel. It has high toughness. But its yield ratio and endurance ratio (i.e., the ratio of fatigue strength to the tensile strength) are low, so that the steel should be used at high strength to obtain adequate yield strength and fatigue strength. The high strength naturally leads to poorer forgeability and machinability which hampers its application to such parts as cited above.